A rotary encoder, a rotary pulse generator, and the like are devices for measuring rotational angles, rotational positions, and the rotating speed of rotors.
A rotary encoder converts an analog measurement of rotational angle to a digital measurement by generating pulse signals with frequencies and amplitudes that are proportional to analog values. The pulse signals can be used to measure a length along which a plate of stainless-steel is cut, to detect a displacement of an arm or the body of a robot, or to choose tools for a machining center through a measurement of rotational angles. The pulse signals can also be used to detect the speed of cars and the engines thereof, and to control rotational angles of dc-motors which are used in tape recorders, facsimile machines, printers and the like. Rotary pulse generators operate in the same manner as rotary encoders, and are preferably used for detecting the rotational angle of low speed rotors.
A general scheme of a conventional rotational angle detecting device is shown in FIG. 1. Reference numeral 4 designates a motor of which the rotational angle is to be detected. A rotating disk 3 is attached to a shaft 5 of the motor 4. A top plane view of the rotating disk 3 is shown in FIG. 2. Photowindows 16 and 26 are formed to have a constant pitch on two circumferences and to allow light to pass through them. Light emitting diodes 11 and 21 and photodetectors 12 and 22, such as photodiodes, are positioned in opposition to each other with the rotating disk 3 placed therebetween.
The light from the diodes 11, 21 passes through the photowindows 16 to reach the photodetector 12 intermittently as the disk 3 rotates. Consequently, the photodetector 12 generates an output current 17 having a sinusoidal waveform as shown in FIG. 3(a). The current 17 is reformed to be pulse trains 18 as shown in FIG. 3(b) through a device such a Schmitt trigger circuit (not shown in the figure).
Similarly, light passes through photowindows 26 and intermittently reaches the photodetector 22 to produce a pulse train 28 as shown in FIG. 3(b). The photodetectors 12 and 22 and the photowindows 16 and 26 are constructed so as to produce pulse trains 18 and 28 with a phase difference of 90 degrees. Such a construction is achieved by arranging the photodetectors 12 and 22 to have equal angular pitch and to have angular positions that differ by about 1/4 of the angular pitch. In this arrangement of photodetectors, the photowindows are constructed so as to illuminate both photodetectors 12, 22 successively. Another construction may be used in which inner and outer photodetectors are arranged on the same angular position, while the photowindows are separated into an inner group and an outer group having equal pitch by angular positions different by about 1/4 of the angular pitch.
FIG. 4 is a circuit diagram of a circuit which receives the pulse trains 18 and 28 of FIG. 3(b) and outputs rotation detecting pulses. A differential circuit 41 generates "1" outputs in response to the rising-edges of the pulse train 18 of FIG. 3(b) to cause an AND gate 42 to generate "1" outputs when the level of the pulse train 28 is "1". The outputs of the AND gate 42, which are supplied to a terminal 43, designate that the disk 3 is rotating in the direction of the arrow A in FIG. 2. Similarly, a differential circuit 44 generates "1" outputs in response to the falling-edges of the pulse train 18 supplied through an inverter 45. An AND gate 46 generates "1" outputs when the levels of the pulses of the pulse train 28 have "1" values. The outputs of the AND gate 46, which are supplied to a terminal 48, designate that the disk 3 is rotating in a direction opposite to that indicated by the arrow A. The outputs of an OR gate 47 designate that the disk 3 is rotating in one or the other direction.
If the number of the photowindows 26 is No, the number of the rotations after the start of the motor 4 is m, and the number of pulses outputted from the terminal 49 in this period is N, the following relation is obtained: EQU N/No=m+(N-mNo)/No (1)
The value of 360.times.(N-mNo) designates an angular difference between the reference point of the motor 4 and that of a stationary plate for counting the number of rotations of the motor 4. The accuracy for measuring the angular difference is expected to be 360/N and, therefore, it is necessary to make No as large as possible in order to improve the accuracy. As for the sensitivities of the photodiodes and the solar batteries conventionally used as the photodetectors 12, 22, an output current density of only about 10-20 uA/cm.sup.2 is obtained under the illuminance of 100 lux. An output current of at least 1 microamperes is required to reduce the expense of the detector and to facilitate its use as a control. Therefore, a lower limit on the size of the photowindows 6 exists from the viewpoint of utility.
If an evaluation is carried out by assuming that conventionally available photoreceivers 12, 22 are used, window areas of 3 mm.times.3 mm are required to obtain an output current of about 1 microamperes. A rotating disk of diameter of about 40 mm requires at least fifteen photowindows. In this case the measuring accuracy of angular difference is 24 degrees.
When a output current of at least 1 microamperes is obtained, it is usual that the output current is converted to a voltage drop of about 10 millivolts through a resistance of about 10 kOhms, which is further amplified to be about one hundred times larger through a conventional amplifier. For a conventional rotating disk which can generate 6000 pulses per revolution, the output current of the photodetector is 1 microampere.times.1/400 (=15/6000). Accordingly, further amplification of the output current of 400 times is required, making it difficult to achieve stable amplification and causing the amplifier circuit to be expensive.
Moreover, if the shaft 5 is attached to a point deviated from the center of the disk 3, the amount of light passing through the photowindows 6 will fluctuate. Therefore, pulsations will appear in the output current of FIG. 3(a), which have a period equal to the cycle of rotation of the disk 3. The pulsations cause distortion of the waveform generated by the reforming circuit, which in turn degrades the accuracy of detection of the rotational angle.
The rotary disk 3 is usually constructed by making slits to be used as the photowindows, or by printing patterns of opaque material on a transparent glass disk to form the photowindows at locations where the patterns do not exist. Irrespective of the process for forming the windows, fluctuations will occur in the shapes, the sizes, and the positions of the photowindows. Hence, a problem arises that the accuracy of detection of the rotational angle is adversely affected by the irregularities in the photowindows. Since the irregularities cause distortions in the waveform of the output currents from the photodetectors, the phase and the shape of individual pulses in the pulse train fluctuate.